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I am using a non inverting amp with B class voltage follower to drive a piezoelectric transducer (used as sonar for fish scanning in boats). I am using trains of 20 sinewave pulses (85KHz), like the ones you can see in the blue image (input).

enter image description here

However, at the output I am getting the green signal, which have undesired residual oscillations with the same frequency. The problem with this, is that the receiver circuit amplify the full incoming wave with a large gain - and there is no way to difference the 20 pulses from the "blank" space. This jamming effect is screwing up the data transmission.

The circuit I build in PCB is this:

enter image description here

I have discovered in simulation that if I decrease the value of the decoupling capacitor, in theory the oscillation tale dissapears. I will try this in the real circuit, but I would like to know which is the correct way to deal with this.

BTW, these parasitic oscillation just happens when the transducer is plugged in the circuit. I am pretty sure isnt related to the operational amplifier.

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  • \$\begingroup\$ you need to dampen. \$\endgroup\$ Mar 26, 2019 at 2:57
  • \$\begingroup\$ Hi. Is there a specific topic I could start researching? Damped systems, 2nd order systems and related stuff are actually a LARGE topic. \$\endgroup\$ Mar 26, 2019 at 2:59
  • \$\begingroup\$ what's the purpose of C1 \$\endgroup\$ Mar 26, 2019 at 3:12
  • \$\begingroup\$ sorry, I meant C1 \$\endgroup\$ Mar 26, 2019 at 3:13
  • \$\begingroup\$ c1 is for decoupling purpose, to filter out any CD offset. I want a pure sinewave in the output \$\endgroup\$ Mar 26, 2019 at 3:14

2 Answers 2

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your output stage has 1.2V of dead-band so when the signal stops it's not driving the output to fixed voltage and so the transducer model continues to ring.

change the output stage to be class AB instead.

or replace C1 with a wire and take the feedback (R4) from Q1 base instead of from U1 output

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  • \$\begingroup\$ I previously had an AB stage, using polarization with 2 diodes. I burnt around 4 transistor trying to set it up and finally gave up. \$\endgroup\$ Mar 26, 2019 at 3:16
  • \$\begingroup\$ @RamiroVargas Your two bases are tied together. So there are times when both BJTs can be OFF and their emitters (sorry) just float, leaving everything on the output side left to its own oscillations. \$\endgroup\$
    – jonk
    Mar 26, 2019 at 3:19
  • \$\begingroup\$ to keep class B change the feedback as my edit suggests. \$\endgroup\$ Mar 26, 2019 at 3:23
  • \$\begingroup\$ @jonk I get it... \$\endgroup\$ Mar 26, 2019 at 3:23
  • \$\begingroup\$ @RamiroVargas I had imagined that folks first determined the resonant frequency of their tranducer and smacked it with a large voltage (perhaps for several cycles worth) to get it ringing. Are you trying to drive a transducer at something other than its resonant frequency? Of course, I've never done anything like this. So that's just my imagination. \$\endgroup\$
    – jonk
    Mar 26, 2019 at 3:26
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A simple fix: remove C1 and C3 and use direct connection; put one of the caps in series with R2. Now add a 220 ohm resistor from the base to the emitter of the transistors. Opamp will drive the transducer directly at crossover and zero out.

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